Surgical intervention is a traumatic experience to the patient. Many surgical procedures require cutting through multiple layers of body tissues including fat, muscles and, sometimes, bones to provide a pathway to a lesion being treated. For example, in a standard appendix operation, the abdominal muscles are cut to expose the appendix. The cut muscles typically take much longer to heal than the injury caused by removing the appendix. In a newer appendix removal operation, using a laproscope, only a single hole is punched through the abdomen to reach the appendix. This type of surgery is part of a growing field that is known as minimally invasive medical procedures.
Minimally invasive medical procedures aim to minimize the trauma to the patient to the minimum necessary for the required therapeutic action. Since most of the trauma in surgery is caused by entering the body, several devices have been developed which can operate within the body and which have a minimal traumatic effect on the body when entering it. For example, endoscopes, which enter through one of the body orifices, for operating in the GI tract, laproscopes which are punched directly into the soft tissue of the body, orthoscopes for operating in joint capsules, vascular catheters for operating in the vascular system and special catheters for the urinary tract. In general, minimally invasive medical procedures are faster, less traumatic to the patient and safer than standard invasive medical procedures.
One example of a minimally invasive procedure is dissolution of a thrombosis using an catheter. Acute myocardial infarcts (heart attacks) and strokes are usually caused by a thrombosis which lodges in a narrowed portion of a blood vessel, blocking it and reducing the supply of oxygen to tissues. In many cases some tissue damage can be averted by promptly removing the thrombosis. In a typical procedure, a catheter is guided through the vascular system to the proximity of the thrombosis. A fibrin dissolving material, such as streptokinase or t-PA enzymes, is injected into the blood vessel and dissolves the thrombosis. In alternative procedures, the thrombosis is cut with a laser beam mounted on the catheter, disintegrated using high power ultrasound channeled through the catheter or compressed against the vessel wall using a balloon. In another minimally invasive medical procedure, a stent is placed in an aneurysm. The stent causes clotting of blood surrounding the stent, so the aneurysm is effectively sealed using the stent. Another type of minimally invasive procedure uses a catheter to inject anti-cancer drugs in proximity to tumors in the brain.
U.S. Pat. No. 4,917,095, the disclosure of which is incorporated herein by reference, describes a minimally invasive procedure for removing gallstones. Gallstones may be formed of two layers, a thin, hard, outer layer which can be disintegrated using an externally generated sonic shockwave and a thick, soft inner layer which can be disintegrated using certain chemicals. In the '095 patent, a catheter or endoscope is brought into the bile ducts and a chemical, which dissolves gallstones, is introduced into the gallbladder. The outer shell of the gallstones is shattered using a sonic shockwave so that the dissolving chemical can disintegrate the soft inner layer. In other procedures, an anti-cancer drug, which is locally injected using a catheter, is made more potent by heating the area using focused ultrasound or microwaves.
U.S. Pat. No. 5,215,680 to D'Arrigo, the disclosure of which is incorporated herein by reference, discloses a method of producing medical-grade lipid coated microbubbles. In addition, the '680 patent discloses that such microbubbles naturally cross capillary walls at most types of tumors. One suggested method of treating tumors is to inject such microbubbles into the blood stream, wait for microbubbles to accumulate in the tumor and irradiate the tumor with high power ultrasound which induces cavitation of the microbubbles. This cavitation completely destroys the tissue in which the microbubbles are accumulated. Another suggested method of tumor destruction is to create microbubbles which encapsulate anti-cancer drugs. Again, when these microbubbles are injected into the bloodstream, the microbubbles accumulate in the tumor, and, after a while, release their anti-cancer drugs.
One method of providing high powered ultrasound at an intra-body location is using focused ultrasound. Usually, ultrasound is focused using a phased array of transmitters. In some systems only the depth of the focal point is controllable, while in others, the focal point can be moved in a plane parallel to the phased array by suitably operating the array. Focused ultrasound, at a sufficient energy density, is used to destroy tissue, especially tumors. However, focused ultrasound has two main limitations. First, the achievable focal spot size is not much smaller than 5 millimeters. Second, the exact location of the focal spot is difficult to determine ahead of time. The acoustic velocity in soft tissue is dependent on the tissue type and, as a result, refraction effects move the focal spot and diffuse it.
One medical procedure, is a liver bypass. Patients which have advanced chirosis of the liver suffer, as a result of blockage of the portal vein, from elevated venous blood pressure, which may cause fatal GI bleeding. In this experimental procedure, a shunt is created between the hepatic vein and the portal vein in the liver to bypass most of the liver. Thus, the venous blood pressure is reduced and GI bleeding eliminated. To create the shunt, a catheter is is inserted into either the portal or the hepatic vein, and a needle is used to probe for the other vein. Since the needle is hollow, when the other vein is found, blood flows through the needle. A stent is guided along the needle to connect the two veins. This procedure is performed using a fluoroscope and is very lengthy, so the amount of radiation exposure of the patient and the surgeon is considerable.
Another experimental medical procedure can be used to aid perfusion in an ischemic heart. This procedure is more fully described in U.S. Pat. No. 5,380,316, the disclosure of which is incorporated herein by reference. In this procedure, a laser tipped catheter is brought into contact with an ischemic portion of the heart and holes, which perforate the heart wall, are drilled into the wall of the heart using the laser. After a short time, perfusion in the ischemic portion improves. It is not at this time clear whether the heart is directly perfused via these holes or whether the trauma caused by drilling the holes encourages the formation of new capillaries. A main concern with this procedure is the perforation of the heart.